Phase change memory structure and manufacturing method for the same

ABSTRACT

Present disclosure provides a semiconductor structure and a method for fabricating a semiconductor device. The semiconductor includes a transistor, a first metallization layer over the transistor, a phase change material over the first metallization layer, a second metallization layer over the phase change material, a heater between the first metallization layer and the second metallization layer and in contact with the phase change material, the heater including a heat insulation shell having a first heat conductivity, wherein the heat insulation shell includes a superlattice structure, and a heat conducting core contacting the heat insulation shell and having a second heat conductivity different from the first heat conductivity.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.15/792,370, filed Oct. 24, 2017, and U.S. application Ser. No.16/532,068, filed Aug. 5, 2019, and claims the benefit thereof under 35U.S.C. 120.

BACKGROUND

Phase change technology is promising for next generation memories. Ituses chalcogenide semiconductors for storing states. The chalcogenidesemiconductors, also called phase change materials, have a crystallinestate and an amorphous state. In the crystalline state, the phase changematerials have a low resistivity, while in the amorphous state they havea high resistivity. The resistivity ratios of the phase change materialsin the amorphous and crystalline states are typically greater than 1000and thus the phase change memory devices are unlikely to have erroneousreading. The chalcogenide materials are stable at certain temperatureranges in both crystalline and amorphous states and can be switched backand forth between the two states by electric pulses. One type of memorydevice that uses the principal of phase change in chalcogenidesemiconductors is commonly referred to as phase change random accessmemory (PCRAM).

PCRAM has several operating and engineering advantages, including highspeed, low power, non-volatility, high density, and low cost. Forexample, PCRAM devices are non-volatile and may be written into rapidly,for example, within less than about 50 nanoseconds. The PCRAM cells mayhave a high density. In addition, PCRAM memory cells are compatible withCMOS logic and can generally be produced at a low cost compared to othertypes of memory cells.

Phase change material is formed between top electrode and bottomelectrode contact. In a reset operation, phase change material may beheated up to a temperature higher than the melting temperature when acurrent passes through it. The temperature is then quickly dropped belowthe crystallization temperature. A portion of the phase change materialis changed to an amorphous state with a high resistivity, thus the stateof the PCRAM cell is changed to a high-resistance state. Region can beset back to the crystalline state by heating up the phase changematerial to a temperature higher than the crystallization temperature,but below the melting temperature, for a certain period.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments are illustrated by way of example, and not bylimitation, in the figures of the accompanying drawings, whereinelements having the same reference numeral designations represent likeelements throughout. The drawings are not to scale, unless otherwisedisclosed.

FIG. 1 is a cross sectional view showing a semiconductor structure, inaccordance with some embodiments of the present disclosure.

FIG. 2 is a cross sectional view showing a semiconductor structure, inaccordance with some embodiments of the present disclosure.

FIG. 3 is a cross sectional view showing a semiconductor structure, inaccordance with some embodiments of the present disclosure.

FIG. 4 is a cross sectional view showing a semiconductor structure, inaccordance with some embodiments of the present disclosure.

FIG. 5 is a cross sectional view showing a semiconductor structure, inaccordance with some embodiments of the present disclosure.

FIG. 6 is a cross sectional view showing a semiconductor structure, inaccordance with some embodiments of the present disclosure.

FIG. 7A is a cross sectional view showing a phase change memorystructure, in accordance with some embodiments of the presentdisclosure.

FIG. 7B is a partial enlarged cross sectional view of Claim 7A, inaccordance with some embodiments of the present disclosure.

FIG. 8A is a cross sectional view showing a phase change memorystructure, in accordance with some embodiments of the presentdisclosure.

FIG. 8B is a partial enlarged cross sectional view of Claim 8A, inaccordance with some embodiments of the present disclosure.

FIG. 9A is a cross sectional view showing a phase change memorystructure, in accordance with some embodiments of the presentdisclosure.

FIG. 9B is a partial enlarged cross sectional view of Claim 9A, inaccordance with some embodiments of the present disclosure.

FIG. 10A is a top view showing relative shapes of a heater and a phasechange material, in accordance with some embodiments of the presentdisclosure.

FIG. 10B is a top view showing relative shapes of a heater and a phasechange material, in accordance with some embodiments of the presentdisclosure.

FIG. 11A is a cross sectional view showing a phase change memorystructure, in accordance with some embodiments of the presentdisclosure.

FIG. 11B is a partial enlarged cross sectional view of Claim 11A, inaccordance with some embodiments of the present disclosure.

FIG. 12 is a top view showing relative shapes of a heater and a phasechange material, in accordance with some embodiments of the presentdisclosure.

FIG. 13A is a cross sectional view showing a phase change memorystructure, in accordance with some embodiments of the presentdisclosure.

FIG. 13B is a partial enlarged cross sectional view of Claim 13A, inaccordance with some embodiments of the present disclosure.

FIG. 14A is a cross sectional view showing a phase change memorystructure, in accordance with some embodiments of the presentdisclosure.

FIG. 14B is a partial enlarged cross sectional view of Claim 14A, inaccordance with some embodiments of the present disclosure.

FIGS. 15, 16, 17, 18, 19, 20A, 20B, and 21 are cross sectional views ofintermediate product in manufacturing a phase change memory structure,in accordance with some embodiments of the present disclosure.

DETAILED DESCRIPTION OF THE DISCLOSURE

The making and using of the embodiments of the disclosure are discussedin detail below. It should be appreciated, however, that the embodimentsprovide many applicable inventive concepts that can be embodied in awide variety of specific contexts. The specific embodiments discussedare merely illustrative of specific ways to make and use theembodiments, and do not limit the scope of the disclosure. Throughoutthe various views and illustrative embodiments, like reference numeralsare used to designate like elements. Reference will now be made indetail to exemplary embodiments illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers are used in thedrawings and the description to refer to the same or like parts. In thedrawings, the shape and thickness may be exaggerated for clarity andconvenience. This description will be directed in particular to elementsforming part of, or cooperating more directly with, an apparatus inaccordance with the present disclosure. It is to be understood thatelements not specifically shown or described may take various forms.Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment. Thus, the appearances of the phrases “in oneembodiment” or “in an embodiment” in various places throughout thisspecification are not necessarily all referring to the same embodiment.Furthermore, the particular features, structures, or characteristics maybe combined in any suitable manner in one or more embodiments. It shouldbe appreciated that the following figures are not drawn to scale;rather, these figures are merely intended for illustration.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

Thermal dissipation is a key parameter for PCRAM structure design sincePCRAM is operated by applying enough current, commonly known as writecurrent, to provide joule heating at the phase change material. Thegreater thermal dissipation at the element providing joule heating, thegreater the write current is required to meet RESET or SET temperature.In the present disclosure, in order to reduce the current required tochange the resistance states of the phase change material, the elementproviding joule heating is designed to have lower thermal dissipation soas to retain the thermal energy more effectively, and hence reduce thelevel of write current.

In some embodiments, the element provides joule heating is a heater. Theheater includes a heat insulation shell and a heat conducting core. Withall other elements identical, the PCRAM with the heater described hereinachieve RESET or SET temperature when applying a write current of 100micro-ampere, as opposed to a write current of 200 micro-ampere in aPCRAM with a conventional heater, i.e., not having the heat insulationshell and heart conducting core described herein.

Referring to FIG. 1 , FIG. 1 is a cross sectional view showing asemiconductor structure 10, in accordance with some embodiments of thepresent disclosure. The semiconductor structure 10 includes asemiconductor chip having a memory region 120 and a peripheral region121. In some embodiments, the memory region 120 is close to a chipcenter than the peripheral region 121. The memory region 120 includes atransistor region 100, a heater 130 over the transistor region 100, anda phase change material 140 over the heater 130.

In some embodiments, the transistor region 100 includes a substrate 101,a source 103, a drain 105, an STI structure 109 in the substrate, and agate 102, a contact plug 107 in an inter-layer dielectric (ILD) 111immediately over the substrate 101. In some embodiments, the substrate101 may include an epitaxial layer formed on the active side, such as anepitaxial semiconductor layer overlying a bulk semiconductor wafer. Insome embodiments, the substrate 101 includes asemiconductor-on-insulator (SOI) structure. For example, the substratemay include a buried oxide (BOX) layer formed by a process such asseparation by implanted oxygen (SIMOX). In various embodiments, thesubstrate 101 includes various p-type doped regions and/or n-type dopedregions, such as p-type wells, n-type wells, p-type source/drainfeatures and/or n-type source/drain features, formed by a process suchas ion implantation and/or diffusion. The substrate 101 may includeother functional features such as a resistor, a capacitor, diode,transistors, such as field effect transistors (FETs). The substrate 101may include lateral insulation features configured to separate variousdevices formed on the substrate 101. The substrate 101 may furtherinclude a portion of a multilayer interconnection (MLI) structure. Themultilayer interconnection structure includes metal lines in a pluralityof metal layers. The metal lines in different metal layers may beconnected through vertical conductive features, which are referred to asvia features. The multilayer interconnection structure further includescontacts configured to connect metal lines to gate electrodes and/ordoped features on the substrate 101. The multilayer interconnectionstructure is designed to couple various devices features (such asvarious p-type and n-type doped regions, gate electrodes and/or passivedevices) to form a functional circuit.

In some embodiments, the transistor region 100 includes heavily dopedregions such as a source 103 and a drain 105 at least partially in thesemiconductor substrate 101. A gate 102 is positioned over a top surfaceof the semiconductor substrate 101 and between the source 103 and thedrain 105 to jointly form a transistor structure 121. FIG. 1 showsplanar transistors having a doped region in the semiconductor substrate101. However, the present disclosure is not limited thereto. Anynon-planar transistor, such as a FinFET structure, can have raised dopedregions. Contact plugs 107 may be formed in a bottom inter-layerdielectric (ILD) 111 adjoined to the top surface of the semiconductorsubstrate 101, and may be electrically coupled to the transistor region100. The bottom ILD 111 above the semiconductor substrate 101 may beformed from a variety of dielectric materials and may, for example, bean oxide (e.g., Ge oxide), an oxynitride (e.g., GaP oxynitride), silicondioxide (SiO₂), a nitrogen-bearing oxide (e.g., nitrogen-bearing SiO₂),a nitrogen-doped oxide (e.g., N₂-implanted SiO₂), silicon oxynitride(Si_(x)O_(y)N_(z)), and the like.

In many instances, shallow trench insulation (STI) structures 109 areprovided to define and electrically isolate adjacent transistors. TheSTI structures 109 are formed in the semiconductor substrate 101.

In many instances, a phase change random access memory (PCRAM)structure, including at least a phase change material, a bottomelectrode and a top electrode of the phase change material 149, isembedded in the metallization layer prepared in a back-end-of-line(BEOL) operation, whereas the transistor region 100 is prepared in afront-end-of-line (FEOL) operation. The PCRAM structure may be embeddedin any position of the metallization layer over the transistor region100, for example, between adjacent metal layers or any of the two metallayers distributed horizontally parallel to a surface of thesemiconductor substrate 101. Throughout the description, the term “metallayer” refers to the collection of the metal lines in the same layer. Onthe other hand, the embedded PCRAM structure can be located between thecontact plug 107 and the first metal layer 150. The number provided forthe metal layer herein is not limiting. In general, people havingordinary skill in the art can understand that the PCRAM can be locatedbetween an N^(th) metal layer and an (N+1)^(th) metal layer, or betweenan N^(th) metal layer and an (N+2)^(th) metal layer, or between anN^(th) metal layer and an (N+M)^(th) metal layer, where N and M areintegers greater than or equal to 1. In an embodiment, N^(th) metallayer includes metal lines surrounded by dielectric layer. Metal linesmay be formed using well-known single damascene process, and may beformed of copper or copper alloys such as AlCu, although other metallicmaterial may also be used.

As shown in FIG. 1 , the heater 130 functions as a bottom electrode tothe phase change material 140, and the metal line 150A of the firstmetal layer 150 functions as a top electrode of the phase changematerial 140. In this embodiment, the heater 130 or the bottom electrodeis in contact with the contact plug 107 for accessing source 103 ordrain 105. The contact plug 107 is surrounded by the bottom ILD 111, anda bottom of the heater 130 or the bottom electrode is in contact withthe bottom ILD 111. The formation of contact plug 107 may includeforming openings in bottom ILD 111, filling the openings, and thenperforming a chemical mechanical polish (CMP). Gate contact plugelectrically connects gate 102 from overlying metal lines and vias (notshown). For simplicity, the gate contact plug for accessing gate 102 isnot shown, although it is also formed simultaneously with contact plug107. Contact plug 107 may be formed of tungsten (W), although otherconductive materials such as silver, aluminum (Al), copper (Cu), AlCu,and the like may also be used or added. Middle ILD 112 over the bottomILD 111 surrounds the heater 130 or the bottom electrode and the phasechange material 140. In the peripheral region 121 of FIG. 1 , thecontact plug 107′ accessing source 103 or drain 105 and the contact plug108′ accessing the gate 102 penetrates through the bottom ILD 111 andthe middle ILD 112.

The phase change material 140 includes commonly used chalcogenidematerials including, but not limited to, one or more of Ge, Te, and Sb,e.g., which may be GeSbTe, NGeSbTe, InGeSbTe, or stoichiometricmaterials thereof.

In FIG. 1 , the heater includes a heat isolation shell 130A composed ofa first material and a heat conducting core 130B composed of a secondmaterial and surrounded by the heat isolation shell 130A. In someembodiments, a first thermal conductivity of the first material issmaller than a second thermal conductivity of the second material. Forexample, the first material may include TaN, which has a thermalconductivity of 3 W/mK, while the second material may include TiN, whichhas a thermal conductivity of 20 W/mK. For another example, the firstmaterial may include TaN, which has a thermal conductivity of 3 W/mK,while the second material may include Ta, which has a thermalconductivity of 57 W/mK. It is shown that the heater 130 has a firstsurface 130C in contact with the phase change material 140 and a secondsurface 130D opposite to the first surface 130C.

In this embodiment, the heat isolation shell 130A encloses a sidewalland a bottom of the heat conducting core 130B, and therefore the firstmaterial and the second material are both exposed from the first surface130C. In other words, the heat conducting core 130B and the heatisolation shell 130A are both in contact with a bottom surface of thephase change material 140 at the first surface 130C. It is shown thatthe entire heat conducting core 130B is in contact with the phase changematerial 140 while only a portion of the heat isolation shell 130A is incontact with the phase change material 140. However, the illustration ofFIG. 1 is not intended to be limiting regarding the interface betweenthe phase change material 140 and the heat conducting core 130B as wellas the heat isolation shell 130A of the heater 130. Other arrangements,such as only the heat conducting core 130B is in contact with the bottomof the phase change material 140, shall be enclosed in the contemplatedscope of present disclosure. On the other hands, as shown in FIG. 1 ,only the first material is exposed at the second surface 130D connectingto the contact plug 107 and the middle ILD 111.

In another embodiment, the heat isolation shell 130A is in contact withonly a sidewall of the heat conducting core 130B, and therefore thefirst material and the second material are both exposed from the firstsurface 130C. In other words, the heat conducting core 130B and the heatisolation shell 130A are both in contact with the phase change material140 at the first surface. On the other hands, the first material and thesecond material are both exposed from the second surface 130D under thescheme that only a sidewall of the heat conducting core 130B issurrounded by the heat isolation shell 130A.

In still another embodiment, the heat isolation shell 130A in contactwith only a bottom of the heat conducting core 130B, and therefore onlythe first material is exposed from the first surface 130C. In otherwords, only the heat conducting core 130B is in contact with the phasechange material 140 at the first surface. On the other hands, only thefirst material is exposed from the second surface 130D under the schemethat heat isolation shell 130A being in contact with only a bottom ofthe heat conducting core 130B.

Referring to FIG. 2 , FIG. 2 is a cross sectional view showing asemiconductor structure 20, in accordance with some embodiments of thepresent disclosure. As shown in FIG. 2 , the phase change material 140is connected to a top electrode 150A, which may be a conductive line ofthe first metal layer 150, and a bottom electrode 140A over the contactplug 107. In some embodiments, bottom electrode 140A is formed of Al,Cu, AlCu, W, or other metallic materials. The formation methods mayinclude commonly used single damascene process, during which middle ILD112 is formed, followed by forming openings, and filling metallicmaterials into the openings. A CMP is then performed to remove excessmetallic materials, leaving bottom electrode 140A. In alternativeembodiments, a blanket metallic material is formed on bottom ILD 111,and is then patterned, leaving bottom electrode 140A. Middle ILD 112 isthen filled. The heater 130 is situated between a bottom of the phasechange material 140 and the bottom electrode 140A and function as abottom electrode contact. In some embodiments, the bottom electrodecontact is a vertical extension of the bottom electrode in the middleILD 112 but with smaller occupying footprint than that of the bottomelectrode 140A. In some embodiments, a width W1 of heater 130 is smallerthan a width W2 of the phase change material 140, therefore only aportion of the phase change material 140 undergoes crystallinity changewhen applying current. In some embodiments, the width W1 of the heateris about 50 to 60 nanometer according to present technology node.Although not illustrate in proportion, in some embodiments, the bottomelectrode 140A has a thickness T1 of from about 100 to 600 Angstrom, theheater has a thickens T2 of from about 50 to about 500 Angstrom, and thephase change material 140 has a thickness T3 of about 300 Angstrom. Insome embodiments, the heater 130 may have a thickness essentially equalto or smaller than that of the bottom electrode 140A.

Referring to FIG. 3 , FIG. 3 is a cross sectional view showing asemiconductor structure 30, in accordance with some embodiments of thepresent disclosure. As shown in FIG. 3 , the phase change material 140is connected to a top electrode 170A, which may be a conductive line ofthe (N+1)^(th) metal layer 170, and a bottom electrode 160A, which maybe a conductive line of the N^(th) metal layer 160 over the first metallayer 150. The PCRAM structure, including the phase change material 140,the top electrode 170A, the bottom electrode 160A, and the heater 130functions as a bottom electrode contact, is previously discussed in FIG.2 and may be referred thereto. The middle ILD 112 between the N^(th)metal layer 160 and the (N+1)^(th) metal layer 170 surrounds the heater130 and the phase change material 140.

Referring to FIG. 4 , FIG. 4 is a cross sectional view showing asemiconductor structure 40, in accordance with some embodiments of thepresent disclosure. As shown in FIG. 4 , the phase change material 140is connected to a top electrode or a heater 130, which may be conductivepad under the first metal layer 150, and a bottom electrode 140A, whichmay be a conductive pad over the conductive plug 107. It is shown thatthe top electrode or the heater 130 has a first surface 130C in contactwith a top surface of the phase change material 140 and a second surface130D opposite to the first surface 130C. In this embodiment, the heatisolation shell 130A encloses a sidewall and a bottom of the heatconducting core 130B, and therefore the first material and the secondmaterial are both exposed from the first surface 130C. However, the heatconducting core 130B is wider than the one shown in FIG. 1 and thus onlythe heat conducing core is in contact with the top surface of the phasechange material 140 at the first surface 130C.

FIG. 5 is a cross sectional view showing a semiconductor structure 50,in accordance with some embodiments of the present disclosure. As shownin FIG. 5 , the phase change material 140 is connected to a topelectrode, which may be a conductive line 150A in the first metal layer150, through a top electrode extension or a heater 130, and a bottomelectrode 140A, which may be a conductive pad in contact with thecontact plug 107 of the transistor region 100. It is shown that the topelectrode extension or the heater 130 has a first surface 130C incontact with a top surface of the phase change material 140 and a secondsurface 130D opposite to the first surface 130C in contact with the topelectrode or the conductive line 150A. In this embodiment, the heatisolation shell 130A encloses a sidewall and a bottom of the heatconducting core 130B, and therefore the first material and the secondmaterial are both exposed from the first surface 130C. The heatconducting core 130B is in contact with a local portion of the topsurface of the phase change material 140 at the first surface 130C.

FIG. 6 is a cross sectional view showing a semiconductor structure 60,in accordance with some embodiments of the present disclosure. As shownin FIG. 6 , the phase change material 140 is connected to a topelectrode, which may be a conductive pad 140A embedded and surrounded bythe middle ILD 112, and a bottom electrode, which may be a conductiveline 150A of the first metal layer 150, through a bottom electrodecontact 130. The top electrode of the conductive pad 140A is further incontact with a via 160A and a metal line 160B of the second metal layer160. It is shown that the bottom electrode contact 130 has a firstsurface 130C in contact with a bottom surface of the phase changematerial 140 and a second surface 130D opposite to the first surface130C in contact with the bottom electrode or the conductive line 150A.In this embodiment, the heat isolation shell 130A encloses a sidewalland a bottom of the heat conducting core 130B, and therefore the firstmaterial and the second material are both exposed from the first surface130C. The heat conducting core 130B is in contact with a local portionof the bottom surface of the phase change material 140 at the firstsurface 130C.

FIG. 7A is a cross sectional view showing a phase change memorystructure 700, in accordance with some embodiments of the presentdisclosure. The phase change memory structure 700 is substantiallyidentical to the memory region 120 of the semiconductor structure 10 inFIG. 1 except for an additional heat assisting layer 130E, composed of athird material and having a third thermal conductivity lower than thesecond thermal conductivity, is disposed at the first surface 130C andin contact with a bottom of the phase change material 140. The heatassisting layer 130E is a portion of the bottom electrode or the heater130. In some embodiments, the third material may be oxygen derivativesor oxynitride derivatives. For example, the heat assisting layer 130Emay include TiO, TiON, TaO, TaON, and the like. In the presentdisclosure, the third material may be considered as a portion of theheat conducting core 130B or a portion of the heat isolation shell 130Ain proximity to the first surface 130C.

In some embodiments, the heat assisting layer 130E is formed byoxidizing the heat conducting core 130B and the heat isolation shell103A from the first surface 130C, and thereby forming the oxidized firstmaterial and oxidized second material. In some embodiments, the oxygencontent in the oxidized first material and oxidized second material isdecreasing starting from the first surface 130C toward the secondsurface 130D. For example, as further illustrated in FIG. 7B, gradedoxygen stoichiometry may be observed starting from the first surface130C toward the heat conducting core 130B and the heat isolation shell130A. In some embodiments, a thickness T4 of the heat assisting layer130E is from about 1 nm to about 5 nm. In some embodiments, if the heatconducting core 130B is composed of TiN and the heat isolation shell130A is composed of TaN, the heat assisting layer 130E can be dividedinto several regions 701, 702, 703, 711, 712, 713, 721, 722, and 723.Region 701 may have a stoichiometry of TiO_(x3)N_(1-x3), region 702 mayhave a stoichiometry of TiO_(x2)N_(1-x2), and region 703 may have astoichiometry of TiO_(x1)N_(1-x1), in which x3 is greater than x2, andx2 is greater than x1. Similarly, region 711 may have a stoichiometry ofTaO_(y3)N_(1-y3), region 712 may have a stoichiometry ofTaO_(y2)N_(1-y2), and region 713 may have a stoichiometry ofTaO_(y1)N_(1-y1), in which y3 is greater than y2, and y2 is greater thany1. In some embodiments, regions 721, 722, 723 may follow thestoichiometry of regions 711, 712, 713. Note the regions may not beevenly spaced but rather follow the diffusion curve of the oxygenapplied in the oxygen plasma or rapid thermal annealing. In someembodiments, the oxygen stoichiometry may be continuously changing orthe oxygen content continuously decreasing along the downward directionand a region with finite thickness having a fixed oxygen stoichiometrymay not be identified.

FIG. 8A is a cross sectional view showing a phase change memorystructure, 800 in accordance with some embodiments of the presentdisclosure. The phase change memory structure 800 is substantiallyidentical to the memory region 120 of the semiconductor structure 20 inFIG. 2 except for an additional heat assisting layer 130E, composed of athird material and having a third thermal conductivity lower than thesecond thermal conductivity, is disposed at the first surface 130C andin contact with a bottom of the phase change material 140. The heatassisting layer 130E is a portion of the bottom electrode contact or theheater 130. In some embodiments, the third material may be oxygenderivatives or oxynitride derivatives. For example, the heat assistinglayer 130E may include TiO, TiON, TaO, TaON, and the like. In thepresent disclosure, the third material may be considered as a portion ofthe heat conducting core 130B or a portion of the heat isolation shell130A in proximity to the first surface 130C.

As further illustrated in FIG. 8B, graded oxygen stoichiometry may beobserved starting from the first surface 130C toward the heat conductingcore 130B and the heat isolation shell 130A. In some embodiments, if theheat conducting core 130B is composed of Ta and the heat isolation shell130A is composed of TaN, the heat assisting layer 130E can be dividedinto several regions 801, 802, 803, 811, 812, 813, 821, 822, and 823.Region 801 may have a stoichiometry of TaO_(x3), region 802 may have astoichiometry of TaO_(x2), and region 803 may have a stoichiometry ofTaO_(x1), in which x3 is greater than x2, and x2 is greater than x1.Alternatively stated, TaO_(x2) can be more oxygen deficient thanTaO_(x3), and TaO_(x1) can be more oxygen deficient than TaO_(x2).Similarly, region 811 may have a stoichiometry of TaO_(y3)N_(1-y3),region 812 may have a stoichiometry of TaO_(y2)N_(1-y2), and region 813may have a stoichiometry of TaO_(y1)N_(1-y1), in which y3 is greaterthan y2, and y2 is greater than y1. In some embodiments, regions 821,822, 823 may follow the stoichiometry of regions 811, 812, 813.

FIG. 9A is a cross sectional view showing a phase change memorystructure 900, in accordance with some embodiments of the presentdisclosure. The phase change memory structure 900 is substantiallyidentical to the memory region 120 of the semiconductor structure 10 inFIG. 1 except for the layering structure of the heat isolation shell130A. In some embodiments, the heat isolation shell 130A includes asuperlattice structure. The superlattice structure is a portion of thebottom electrode or the heater 130. Superlattice structures, in general,are known and typically comprise a composite made of alternatingultrathin layers of different materials. By the appropriate choice ofmaterials, a superlattice having a heat conduction structure and othercharacteristics can be produced. In accordance with the presentinvention, a superlattice structure includes m monolayers of a firstmaterial alternating with n monolayers of a fourth material with a pairof monolayers defining a superlattice period and each of the materialshaving a relatively smooth interface therebetween. In the presentdisclosure, a monolayer of material includes a single, closely packedlayer of atoms of such material. For example, a TiN monolayer includes asingle, closely packed layer of titanium and nitrogen atoms arranged ina preferred unit cell structure. The fourth material having a fourththermal conductivity. The fourth thermal conductivity may be greaterthan or smaller than the second thermal conductivity of the secondmaterial because the low dimension of superlattice structure renderingmaterial property of bulk material not the only determination factor forthermal conductivity design.

The thermal transport by phonons, however, can also be controlledthrough the proper design of the superlattice unit cell. Thesuperlattice thermal conductivity tensor is anisotropic, with thein-plane thermal conductivity often being several factors greater thanthe cross-plane value. This characteristic may lead to the use ofsuperlattices in directional heat transfer applications where a materialthat can simultaneously insulate in one direction and channel heat inanother is desired. As shown in FIG. 9A, the portion of heat isolationshell 130A at a side of the heat conducting core 130B is enlarged inFIG. 9B. In this embodiment, a layer 1301A of first material and a layer1302A of a fourth material form one superlattice period. As shown inFIG. 9B, 4 superlattice periods composed of alternating first materialand fourth material are shown. A layer thickness D1 of the firstmaterial may be equal to a layer thickness D4 of the fourth material. Insome embodiments, D1 and D4 are both 1 nanometer. However, D1 and D4 maynot be equally thick. As discussed previously, the thermal conductivityin the in-plane direction A may be several factors greater than that inthe cross-plane direction B. In other words, the heat generated in theheat conducting core 103B is harder to dissipate to the surrounding, forexample, the middle ILD 112, via the superlattice structure of the heatisolation shell 130A along the cross-plane direction.

In one embodiment, when only one period of alternating first materialand fourth material is in the heat isolation shell 130A, the fourththermal conductivity may be smaller than the second thermalconductivity. For example, the first material may be TaN, having athermal conductivity of 3 W/mK, the second material may be Ta, having athermal conductivity of 57 W/mK, and the fourth material may be TiN,having a thermal conductivity of 20 W/mK.

FIG. 10A and FIG. 10B are top views showing relative shapes of a heater130 and a phase change material 140, in accordance with some embodimentsof the present disclosure. In FIG. 10A, a footprint of the phase changematerial 140 only lands on the heat conducting core 130B, whereas inFIG. 10B, a footprint of the phase change material 140 lands on both theheat conducting core 130B and the heat isolation shell 130A.

FIG. 11A is a cross sectional view showing a phase change memorystructure 1100, in accordance with some embodiments of the presentdisclosure. The phase change memory structure 1100 is substantiallyidentical to the memory region 120 of the semiconductor structure 20 inFIG. 2 except for the layering structure of the heat isolation shell130A. In some embodiments, the heat isolation shell 130A includes asuperlattice structure. The superlattice structure is a portion of thebottom electrode contact or the heater 130. As shown in FIG. 11A, theportion of heat isolation shell 130A at a side of the heat conductingcore 130B is enlarged in FIG. 11B. In this embodiment, a layer 1301A offirst material and a layer 1302A of a fourth material form onesuperlattice period. As shown in FIG. 11B, 4 superlattice periodscomposed of alternating first material and fourth material are shown. Alayer thickness D1 of the first material may be equal to a layerthickness D4 of the fourth material. In some embodiments, D1 and D4 areboth 1 nanometer. However, D1 and D4 may not be equally thick. Asdiscussed previously, the thermal conductivity in the in-plane directionA may be several factors greater than that in the cross-plane directionB. In other words, the heat generated in the heat conducting core 103Bis harder to dissipate to the surrounding, for example, the middle ILD112, via the superlattice structure of the heat isolation shell 130Aalong the cross-plane direction.

FIG. 12 is a top view showing relative shapes of a heater 130 and aphase change material 140, in accordance with some embodiments of thepresent disclosure. In FIG. 12 , a footprint of the heater 130 lands ona narrowest portion of the footprint of the phase change material 140.Due to the relatively great resistance of narrow portion, the phasechange may occur at narrow portion. In other embodiments, the footprintof the phase change material 140 may be a stripe with equal width.

FIG. 13A is a cross sectional view showing a phase change memorystructure 1300, in accordance with some embodiments of the presentdisclosure. The phase change memory structure 1300 is substantiallyidentical to the phase change memory structure 900 in FIG. 9A except foran additional heat assisting layer 130E, composed of a third materialand having a third thermal conductivity lower than the second thermalconductivity, is disposed at the first surface 130C and in contact witha bottom of the phase change material 140. The heat assisting layer 130Eis a portion of the bottom electrode or the heater 130. In someembodiments, the third material may be oxygen derivatives or oxynitridederivatives. In some embodiments, the heat assisting layer 130E isformed by oxidizing the heat conducting core 130B and the heat isolationshell 103A from the first surface 130C, and thereby forming the oxidizedfirst material and oxidized second material. In some embodiments, theoxygen content in the oxidized first material and oxidized secondmaterial is decreasing starting from the first surface 130C toward thesecond surface 130D. For example, as further illustrated in FIG. 13B,graded oxygen stoichiometry may be observed starting from the firstsurface 130C toward the heat conducting core 130B and the heat isolationshell 130A. As discussed previously, the graded oxygen stoichiometry ofregion 1301 may follow regions 701, 702, 703 of FIG. 7B, and the gradedoxygen stoichiometry of regions 1310 and 1320 may follow regions 711,712, 713, and 721, 722, 723, respectively, of FIG. 7B. The reductiontrend of oxygen content in different sublayers of the heat assistinglayer 130E can be referred to the description of FIG. 7B and is notrepeated here for brevity.

FIG. 14A is a cross sectional view showing a phase change memorystructure 1400, in accordance with some embodiments of the presentdisclosure. The phase change memory structure 1400 is substantiallyidentical to the phase change memory structure 1100 in FIG. 11A exceptfor an additional heat assisting layer 130E, composed of a thirdmaterial and having a third thermal conductivity lower than the secondthermal conductivity, is disposed at the first surface 130C and incontact with a bottom of the phase change material 140. The heatassisting layer 130E is a portion of the bottom electrode extension orthe heater 130. In some embodiments, the third material may be oxygenderivatives or oxynitride derivatives. In some embodiments, the heatassisting layer 130E is formed by oxidizing the heat conducting core130B and the heat isolation shell 103A from the first surface 130C, andthereby forming the oxidized first material and oxidized secondmaterial. In some embodiments, the oxygen content in the oxidized firstmaterial and oxidized second material is decreasing starting from thefirst surface 130C toward the second surface 130D. For example, asfurther illustrated in FIG. 14B, graded oxygen stoichiometry may beobserved starting from the first surface 130C toward the heat conductingcore 130B and the heat isolation shell 130A. As discussed previously,the graded oxygen stoichiometry of region 1401 may follow regions 801,802, 803 of FIG. 8B, and the graded oxygen stoichiometry of regions 1410and 1420 may follow regions 811, 812, 813, and 821, 822, 823,respectively, of FIG. 8B. The reduction trend of oxygen content indifferent sublayers of the heat assisting layer 130E can be referred tothe description of FIG. 8B and is not repeated here for brevity.

FIGS. 15, 16, 17, 18, 19, 20A, 20B, and 21 are cross sectional views ofintermediate product in manufacturing a phase change memory structure,in accordance with some embodiments of the present disclosure. In FIG.15 , a transistor region 100 is formed in a substrate 101 and a bottomILD 111. A contact plug 107 accessing the drain 105 of the transistorregion 100 is surrounded by the bottom ILD 111 and is in contact withthe first metal layer 150. Metal lines 150A in the first metal layer 159ma be formed using well-known single damascene process, and may beformed of copper or copper alloys such as AlCu, although other metallicmaterial may also be used. In some embodiments, the metal line 150A ofthe first metal layer 150 is a bottom electrode for the PCRAM structure.In FIG. 16 , middle ILD 112 is formed using a single damascene process.A contact trench 130′ is patterned in the middle ILD 112 with a desiredwidth W1. The width W1 is determined based on a width W2 of a phasechange material to be subsequently formed. The width W1 can be widerthan the width W2 so as to cover the entire phase change material. Onthe other hand, the width W1 can be narrower than the width W2 so as tocontact a local portion of the phase change material.

In FIG. 17 , a heat insulation shell 130A or a heat insulation layer isconformably deposited in the trench 130′ and over the top surface of themiddle ILD 112. In some embodiments, the thickness T5 of the heatinsulation shell 130A or the heat insulation layer is about 10 to 100Angstrom. As previously discussed, the heat insulation layer is composedof a first material having a first thermal conductivity. Therefore, onlya single layer of first material is deposited using, for example,physical vapor deposition (PVD) or chemical vapor deposition (CVD). Insome embodiments, the heat insulation layer is composed of severalmaterials having an effective first thermal conductivity. When theseveral materials of the heat insulation layer are arranged in analternating fashion with repetition order, the heat insulation layerincludes a superlattice having an effective first thermal conductivity.In this case, a superlattice structure is deposited using, for example,atomic layer deposition (ALD). In FIG. 18 , a heart conductive core 130Bor a heat conductive material is deposited to fill up the trench 130′and overflow to the top of the middle ILD 112 as well as the heatinsulation layer. As previously discussed, the heat conductive materialis composed of a second material having a second thermal conductivitygreater than the first thermal conductivity. In FIG. 19 , the excessheat insulation layer and the heat conductive material are removed froma top of the middle ILD 112 through a planarization operation, forexample, a chemical mechanical polishing (CMP), resulting a firstsurface 130C simultaneously exposing the heat conducting core 130B, theheat isolation shell 130A, and the middle ILD 112. In other words, thefirst surface 130C is a coplanar surface for the heat conducting core130B, the heat isolation shell 130A, and the middle ILD 112.

Two subsequent operations can follow the operation illustrated in FIG.19 , that is, FIG. 20A and FIG. 20B. If FIG. 20A is chosen, a phasechange material 140 is formed over and in contact with the first surface130C. The phase change material 140 may be formed using well-knownsingle damascene process by forming an opening with a predeterminedwidth W2 in the middle ILD 112, filling the opening with phase changematerial 140, and removing excess phase change material 140. Phasechange material 140 may include commonly used chalcogenide materialsincluding, but not limited to, one or more of Ge, Te, and Sb, e.g.,which may be GeSbTe, NGeSbTe, InGeSbTe or stoichiometric materials. IfFIG. 20B is chosen, an oxygen treatment 200 is performed over the firstsurface 130C before the formation of the phase change material 140 overthe first surface 130C, as illustrated in FIG. 21 .

The oxygen treatment 200 illustrated in FIG. 20B is applied to thecoplanar surface or the first surface 130C composed of at least the heatconducting core 130B and the heat isolation shell 130A. In someembodiments, the oxygen treatment 200 may include a plasma oxidationusing atomic oxygen generated in a plasma. In one embodiment, oxidizingwith a plasma can be accomplished with atomic oxygen generated in aKrypton/oxygen plasma. Alternately, the plasma can be generated usinghelium. In one embodiment, prior to the plasma oxidation process, oxygenis supplied during the planarization operation to provide a small seedamount of oxygen into the film prior to oxidation. This initial supplyof oxygen is provided using an ionizer ring. The mixed plasma processgenerates atomic oxygen or oxygen radicals in contrast to molecularoxygen or O₂ used in conventional thermal oxidation. The atomic oxygenis introduced to the heat conducting core 130B and the heat isolationshell 130A from the exposed first surface 130C, creating an oxideportion. The atomic oxygen continues to react with the layer and createsan oxidation interface. As the reaction progresses, atomic oxygendiffuses through the oxide portion and reacts at the oxidation interfaceuntil a desired thickness of heat assisting layer 130E is formed.

In some embodiments, the oxygen treatment 200 may include a rapidthermal annealing (RTA) operation which is carried out in an oxidizingambient such as oxygen, NO, N₂O, NH₃ and other like oxidizing ambient.Combinations of these oxidizing ambient are also contemplated herein.The oxidizing ambient or combination thereof may also be used inconjunction with a carrier gas (or mixture of carrier gases) such ashelium, argon, nitrogen or other like inert carrier gases. Combinationsof the above rapid thermal processes such as RTA, rapid thermalnitridation (RTN), and rapid thermal oxynitridation (RTON) are alsocontemplated herein. The RTA, RTN or RTON operations are carried out ata temperature of about 400° C. or above and for a time period of fromabout 180 seconds or less. More specifically, the RTA, RTN or RTONoperations are carried out at a temperature of from about 600° to about900° C. for a time period of from about 5 to about 60 seconds. Othertemperatures and times that are capable of forming an interfacial oxide,oxynitride and/or nitride layer on a surface of heat conducting core130B and the heat isolation shell 130A can also be used in the RTA, RTNor RTON operations. The RTO, RTON or RTN process is carried out in amotionless chamber, or in a chamber in which rotation of the substrateis possible. Chamber rotation is preferred in the present inventionsince it aids in improving growth rate uniformity of the heat assistinglayer 130E.

The heat assisting layer 130E formed using oxygen plasma operation mayhave a thinner surface oxide/oxynitride compared to the one formed usingRTA, RTN, RTON operations. In some embodiments, oxygen plasma operationhas a better thickness control of the heat assisting layer 130E than therapid thermal annealing operations.

Some embodiments provide a phase change memory structure, including atransistor region, a phase change material over the transistor region, aheater over the transistor region and in contact with the phase changematerial, and a dielectric layer surrounding the heater and the phasechange material. The heater includes a first material having a firstthermal conductivity, the first material disposed at a periphery of theheater, and a second material having a second thermal conductivitygreater than the first thermal conductivity, the second materialdisposed at a center of the heater.

Some embodiments provide a semiconductor device, including a transistor,a first metallization layer over the transistor, a phase change materialover the first metallization layer, a second metallization layer overthe phase change material, a dielectric layer between the firstmetallization layer and the second metallization layer, surrounding thephase change material, and a heater in the dielectric layer and incontact with the phase change material. The heater includes a heatinsulation shell and a heat conducting core, the heat insulation shellobstructing heat dissipation from the heat conducting core.

Some embodiments provide a method for manufacturing a phase changememory structure, the method includes forming a bottom electrode,forming a dielectric layer over the bottom electrode, the dielectriclayer patterned with a contact trench, depositing a heat insulationlayer having an effective first thermal conductivity in a sidewall and abottom of the contact trench, filling the contact trench with a heatconductive material having a second thermal conductivity greater thanthe effective first thermal conductivity, planarizing the firstmaterial, the second material, and the dielectric layer to form acoplanar surface of the first material, the second material, and thedielectric layer, and forming a phase change material over and incontact with the coplanar surface.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims. For example,many of the processes discussed above cancan be implemented in differentmethodologies and replaced by other processes, or a combination thereof.

Moreover, the scope of the present application is not intended to belimited to the particular embodiments of the process, machine,manufacture, composition of matter, means, methods and steps describedin the specification. As one of ordinary skill in the art will readilyappreciate from the disclosure of the present invention, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed, that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present invention. Accordingly, the appended claims are intended toinclude within their scope such processes, machines, manufacture,compositions of matter, means, methods, or steps.

What is claimed is:
 1. A semiconductor device, comprising: a transistor;a first metallization layer over the transistor; a phase change materialover the first metallization layer; a second metallization layer overthe phase change material; a heater between the first metallizationlayer and the second metallization layer and in contact with the phasechange material, the heater comprising: a heat insulation shell having afirst heat conductivity, wherein the heat insulation shell comprises asuperlattice structure; and a heat conducting core contacting the heatinsulation shell and having a second heat conductivity different fromthe first heat conductivity.
 2. The semiconductor device of claim 1,wherein the first thermal conductivity of the heat insulation shell issmaller than the second thermal conductivity of the heat conductingcore.
 3. The semiconductor device of claim 1, wherein the heater is incontact with the phase change material at a first surface comprising theheat insulation shell and the heat conducting core.
 4. The semiconductordevice of claim 1, wherein a width of the heater is narrower than awidth of the phase change material.
 5. The semiconductor device of claim1, wherein the heat insulation shell comprises tantalum nitride.
 6. Thesemiconductor device of claim 1, wherein the heat conducting corecomprises titanium nitride or tantalum.
 7. The semiconductor device ofclaim 1, further comprising an insulation layer laterally surroundingthe heater.
 8. The semiconductor device of claim 7, wherein a thermalconductivity of the insulation layer is different from the secondthermal conductivity of the heat conducting core.
 9. A semiconductordevice, comprising: a transistor; a first metallization layer over thetransistor; a phase change material over the first metallization layer;a second metallization layer over the phase change material; a heaterbetween the first metallization layer and the second metallization layerand in contact with the phase change material, the heater comprising: aheat insulation shell having a first heat conductivity; and a heatconducting core contacting the heat insulation shell and having a secondheat conductivity different from the first heat conductivity; and aninsulation layer contacting the heat insulation shell, wherein theinsulation layer comprises graded oxygen derivatives of the heatinsulation shell and the heat conducting core.
 10. The semiconductordevice of claim 9, wherein the first thermal conductivity of the heatinsulation shell is smaller than the second thermal conductivity of theheat conducting core.
 11. The semiconductor device of claim 9, whereinthe heater is in contact with the phase change material at a firstsurface comprising the heat insulation shell and the heat conductingcore.
 12. The semiconductor device of claim 9, wherein a width of theheater is narrower than a width of the phase change material.
 13. Thesemiconductor device of claim 9, wherein a thermal conductivity of theinsulation layer is lower than the second thermal conductivity of theheat conducting core.
 14. The semiconductor device of claim 9, whereinthe heat insulation shell comprises a superlattice structure.
 15. Thesemiconductor device of claim 9, wherein the heat insulation shellcomprises tantalum nitride.
 16. The semiconductor device of claim 9,wherein the heat conducting core comprises titanium nitride or tantalum.17. A method for manufacturing a phase change memory structure,comprising: forming a first metal layer; forming a dielectric layer overthe first metal layer, the dielectric layer patterned with a contacttrench; depositing a heat insulation layer having an effective firstthermal conductivity in the contact trench; filling the contact trenchwith a heat conductive material having a second thermal conductivitygreater than the effective first thermal conductivity; forming a phasechange material over the heat conductive material and the heatinsulation layer; forming an insulation layer derived from the heatinsulation layer and the heat conductive material prior to forming thephase change material, wherein forming the insulation layer comprisesperforming an oxidation operation; and forming a second metal layer overthe phase change material.
 18. The method of claim 17, wherein formingthe first metal layer comprises forming a first metallization layer. 19.The method of claim 17, wherein the heat insulation layer comprisestantalum nitride.
 20. The method of claim 17, wherein the heatconductive material comprises titanium nitride or tantalum.